a tax on money transfers wired out of the country by noncitizens would build a nice wall
It doesn't need to be much. Just a small "user fee" on remittances. Less than FEDEX would charge to deliver to Mexico.
"Mexicans sent home $26.1 billion from January to November 2017, according to figures released Tuesday by the central bank of Mexico." California charges a 7.25% sales tax. 7.25% x $26.1 billion is $1.9575 billion. Over 20 years that's nearly $40 billion, and that's if there is no growth.
(Department of Homeland Security said the wall could cost about $21.6 billion, not including maintenance. There are other figures from the left, apparently pulled out of their collective ***. Trump is a builder and if anyone can do this at a reasonable cost, he can.)